In our last post we saw that God freely made us to have fellowship with Him. But that is not the whole story because of sin. Man turns his back on God. But God in His love acted to save us through Christ (John 3:16):
He calls together all men, scattered and divided by sin, into the unity of his family, the Church. To accomplish this, when the fullness of time had come, God sent his Son as Redeemer and Savior. In his Son and through him, he invites men to become, in the Holy Spirit, his adopted children and thus heirs of his blessed life. [CCC §1]
And how does He accomplish this call? In and through His Church:
Go, therefore, make disciples of all nations; baptise them in the name of the Father and of the Son and of the Holy Spirit, and teach them to observe all the commands I gave you. And look, I am with you always; yes, to the end of time. [Matthew 28:19–20, NJB]
See also CCC §2–3:
2 So that this call should resound throughout the world, Christ sent forth the apostles he had chosen, commissioning them to proclaim the gospel: “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age.” Strengthened by this mission, the apostles “went forth and preached everywhere, while the Lord worked with them and confirmed the message by the signs that attended it.”
3 Those who with God’s help have welcomed Christ’s call and freely responded to it are urged on by love of Christ to proclaim the Good News everywhere in the world. This treasure, received from the apostles, has been faithfully guarded by their successors.
The Church and the Gospel go together. God does not send one without the other. And that this Church is a visible institution rather than an unknowable group of people is implied by the fact that its sacrament of initiation is something visible: Holy Baptism. So Christ intended His Church to be something that the world could see.
The necessity of a visible Church seems clear: how else is one to know who speaks for God? And this necessity is declared by the Catechism of the Council of Trent (and by Scripture) as well:
But, as faith cometh by hearing, it is evident how necessary is the faithful labour and ministry of a legitimate teacher, at all times, towards attaining to eternal salvation, seeing it is written: How shall they hear without a preacher? And how shall they preach, except they be sent? [Question II, quoting Romans 10:14–15]
In short: we need the Gospel so that we may be saved, so that we may see our Lord face to face. And we need the Church for the sake of the Gospel.